In commercial and residential power distribution systems, power is tapped from one or more main power lines and delivered to a building Typically, each building contains several different circuits which receive the delivered power and distribute the power as needed to electrical loads associated with each circuit. Each of these circuits usually contains a protective device which interrupts the power delivered by the circuit when the electrical loads draw more power through the circuit than the circuit was designed to safely handle. Presently, circuit breakers are commonly chosen as the protective device for use in power distribution systems.
Since these circuits typically branch out from the location at which power is delivered to the building, the circuit breakers are centrally situated near this location. The circuit breakers are mounted in a case which encompasses the circuit breakers and any proximate terminals and bare wires associated with the power distribution system. The case is sealed to prevent people and objects from accidentally making contact with the power distribution lines attached to each circuit breaker. Moreover, the circuit breaker cases are supplied with removable access covers so that the circuit breakers may be serviced or replaced.
For safety reasons Underwriters Laboratories prohibits access to internal components when the access cover is in place. Therefore, the access cover is typically screwed into threaded inserts in the case to provide a tight seal between the access cover and the case. While this type of attachment certainly meets the requirements of Underwriters Laboratories, usually a number of screws must be removed in order to detach the cover from the case.
Typically, the access covers and cases are made from an electrically insulating material, such as plastic. This helps prevent an accidental contact of a live wire with the access cover or case from electrifying the access cover or the case. It is also preferable to use fasteners that are made from an insulating material to attach the access cover to the case. But plastic or composite screws can be easily damaged by misthreading and are often brittle.
Accordingly, there is a need for an alternative means for providing a seal in accordance with Underwriters Laboratories' safety requirements, while allowing the access cover to be easily removed from the circuit breaker case.